Earthquake Activities along the Lampang-theon Fault Zone ... · University, 1-1 Tegatagakuen, Akita...

Journal of Applied Sciences Research 5(2): 168-180, 2009

© 2009, INSInet Publication

Earthquake Activities along the Lampang-theon Fault Zone, Northern Thailand:Evidence from Paleoseismological and Seismicity Data

Santi Pailoplee, Isao Takashima, Suwith Kosuwan and Punya Charusiri1 2 3 1

Earthquake and Tectonic Geology Research Unit (EATGRU), c/o Department of Geology, Faculty of1

Science, Chulalongkorn University, Bangkok 10330, Thailand

Research Institute of Materials and Resources, Faculty of Engineering and Resources Science, Akita2

University, 1-1 Tegatagakuen, Akita 010-8502, Japan

Environmental Geology Division, Department of Mineral Resources, Rama VI, Bangkok 10400,3

Thailand

Abstract: Within the northern part of Thailand lies the Lampang-Theon fault zone (LTFZ), one of the

most important active fault zones in the region, as testified by historical and instrumental seismicity.

However, the LTFZ has received no detailed study. In this work, two fault segments; the Ton Ngoon and

Ban Mai segments near the Lampang province were clarified by remote sensing and trench-log

interpretations, and chronological investigation including seismicity investigation. From trench-log

interpretations, the Ton Ngoon segment revealed two obvious palaeo-earthquakes which, by

thermoluminescence (TL) and accelerator mass spectrophotometric (AMS) radiocarbon dating of samples,

occurred approximately 1,800 and 3,500 years ago. The rate of the last fault movement was 0.18 mm/yr

with a recurrence interval for large earthquakes of 1,700 years. The Ban Mai segment revealed a single

main palaeo-earthquake which, by TL-dating, occurred some 3,800 years ago with a slip rate of

approximately 0.06 mm/yr. The current seismicity investigation (i.e. b value) of the LTFZ and the Phrae

fault zone (PFZ) revealed a lower b value for the LTFZ which can generate more earthquake activity than

the PFZ. Sliding time window analysis, containing 30 earthquake events and five event shifts, revealed

temporal variations in the b value in the Lampang-Theon fault zone. Three significant drops in the LTFZ

b value down to 1-1.2 coincide with the occurrence of Mw 3-4 earthquakes. Thus these conditions

successfully analyze the b(t) in the LTFZ and can be applied for forecasting the occurrence of small to

intermediate earthquakes in other specific areas.

Key words: Lampang-Theon fault zone; Thermoluminescence dating; Earthquake catalogue; b value;

Thailand.

INTRODUCTION

Northern Thailand is dominated by a large number

of active fault zones such as Mae Chan , Pua ,[1] [2]

Phrae , Mae Tha and Mae Kuang , and Lampang-[3] [4]

Thoen fault zones. These fault zones have revealed[5]

neotectonic activity, possibly in response to the

continuous northward subduction of the Indian plate

beneath the Eurasian plate . The left-lateral Mae[2 ,6]

Chan fault zone indicates the latest fault movement of

1.5 ka, based on Thermoluminescence (TL) and

accelerator mass spectrometry (AMS) radiocarbon

dating . In addition, ground shaking with an intensity[1]

of VII or greater was reported in A.D. 460, 534, and

1715 . The Pua fault zone is a north-striking, west-[7]

dipping normal fault bounding the eastern margin of

the Tertiary Pua basin . The most prominent tectonic[2]

geomorphology along this fault zone is marked by the

steep, west-facing escarpment with triangular facets and

wineglass canyons. These morphotectonic evidences

imply a minimum vertical displacement rate of about

0.6 mm/yr . Within the phrae fault zone (PFZ),[2]

paleoseismological evidence in the southeastern part of

the Phrae basin (the black cross in Fig. 1a), using TL

dating to constrain the chronological data of the fault,

suggested that the Phrae segment (southeastern part of

Phrae basin) is potentially active with a mean

recurrence period of 0.9 Ma and a maximum slip rate

of 0.06 mm/yr . For the Mae Tha and Mae Kuang[3]

fault zones, analysis of the fault traces using enhanced

Corresponding Author: Punya Charusiri, Earthquake and Tectonic Geology Research Unit (EATGRU), c/o Department ofGeology, Faculty of Science, Chulalongkorn University, Bangkok 10330, ThailandTel. (66) 2218-5456 Fax : (66) 2218-5464E-mail : [email protected], [email protected]

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J. App. Sci. Res., 5(2): 168-180, 2009

remote-sensing data lead to the suggestion that the Mae

Tha forms a 140 km-long NW-trending trace to the

east of the Chiang Mai basin . Hot springs, which[4 ,8]

represent the present-day tectonic activities, are located

in the southern part of the fault. Along the northern

part, the right-lateral Mae Tha fault trace is apparently

truncated by the NE-SW Mae Kuang fault, which itself

is the trace of a Cenozoic, and perhaps active,

predominantly left-lateral fault with a total slip of

approximately 3.5 km . [4]

The Lampang-Thoen fault zone (LTFZ) (Fig. 1a)

is one of the large fault systems in northern Thailand.

Strong activities were reported along several fault

segments during the Late Quaternary period . The[9]

LTFZ activities control river valleys and are associated

with rifting basins which are bounded by this fault

zones . The detailed study of Quaternary faults in the[10]

LTFZ using remote-sensing interpretation has[5 ]

attracted considerable attention because this fault zone

shows prominent morphotectonics associated with the

faults (i.e. triangular facet and shutter ridge) and is

located close to the populated Lampang provinces (Fig.

1). However, the earthquake hazard parameters (e.g. the

maximum earthquake magnitude, rate of fault slip,

recurrence interval of large earthquakes, the possible

rupture area, including the seismic activities; a and b

values from the Gutenberg-Richter law) of this fault

zone are still controversial, mainly due to insufficient

reliable chronological information about the fault and

the current seismicity activity. We derived these

earthquake hazard parameters along the LTFZ using

both paleoseismological and seismicity approaches. The

main aim of this study was to clarify the earthquake

activities and derive their earthquake hazard parameters,

which are necessary for seismic hazard analysis, along

the LTFZ and nearby areas.

2. Paleoseismological Investigation: In order to derive

the recurrence times between large earthquakes along

the LTFZ, the paleoseismological investigations were

investigated for two prominent fault segments near the

Lampang province; the Ton Ngoon (no. 1 in Fig. 1)

and Ban Mai (no. 2 in Fig. 1) segments.

2.1 Ton Ngoon Segment: Based on enhanced remote

sensing interpretation, the 9 km-long Ton Ngoon

segment is located in the southeastern part of the

Lampang province. The fault lies in an almost E-W

direction cut across the Permo-Triassic rhyolite and

Quaternary colluvium. The series of triangular facets

indicate a normal movement in this fault segment (no.

1 in Fig. 1 b). We excavated a 22-m long and 2-m

wide trench, up to 2.5 m deep on the northern side of

the trench, in a N-S direction across the expected fault

trace at the eastern portion of the Ton Ngoon segment,

at a latitude of 18 12’ N and alongitude of 99 27’ E,û û

(Fig. 1). Based on trench logging, the stratigraphy

shows three colluvium deposits and one unit of in situ

weathered basement rock related to fault movements

(Fig. 2). The bottommost layer (unit D) in the trench

wall, being weathered rhyolite with abundant fractures

and zones of alterations, underlies the thin bed of

gravel to silt layer (unit C). The third layer from the

bottom (unit B) was comprised of fine silt to coarse

sand layer which was only deposited in the northward

section of the trench. The topmost layer (unit A) is

sandy silt mixed with some gravel. Beside a large

number of fractures in the unit D, two earthquake-

induced offsets of sediment layers were recorded in the

trench. F1 (in Fig. 2) is the series of sediment offset

cut through unit D. F2 (in Fig. 2) is the fault which

cuts the sediment layer from the bottom through units

D, C, and B respectively.

2.2 Ban Mai Segment: The NE-SW Ban Mai segment

is 29-km long and extends southward through the

eastern boundary of the Lampang basin (Fig. 1 b). The

Ban Mai segment cuts across the Triassic sand to silt

and was covered by colluvium and terrace deposits in

the Quaternary period . Several streams crossing the[5]

Ban Mai segment exhibit wine-glass canyons which

indicate uplifting of the footwall in a normal fault

system . The series of triangular facets and shutter[2]

ridges are conspicuous along the fault trace (Fig. 1 b).

We excavated a 8-m long, 1.5-m wide, and 2.5-m deep

trench in the southern part of the segment, at latitude

18 13’ N and longitude 99 32’ E, (Fig. 1), from whichû û

seven detailed stratigraphic sequences could be

identified in both side walls of the trench (A – G in

Fig. 3). All seven of the layers are matrix supported by

clay. The bottommost layer (unit G) is gravel mixed

with yellowish brown clay and is overlain by the

gravel bed (unit F). Thereafter, gravel mixed with

yellowish brown clay (same as unit G) was deposited

and classified as unit E. The next upper layer (unit D)

is the lateritic gravelly clay overlaid by sandy clay

(unit C), gravelly clay (unit B), and top soil layers

(unit A) of sand, silt or clay, respectively. Besides

some sediment fractures that are dominate in unit E,

one obvious fault can be categorized which cuts across

sediment units G, F, E and up to D (Fig. 3). This

prominent fault vertically shifts the sediment layers and

illustrates normal faulting.

3. Geochronological Investigations: As mentioned

above, chronological data and in particular the date

which individual earthquake events occurred and the

rate of the fault slip are of significant importance for

the interpretation of seismic hazard parameters. After

investigating the sediment layer associated with the

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Fig. 1: Map of Northern Thailand showing the locations of possible active faults, earthquake distribution and

location of Ton Ngoon (no. 1) and Ban Mai (no. 2) trenching; close up satellite image showing Ton

Ngoon (no. 1) and Ban Mai (no.2) Faults and trenching location (white triangular).

fault, we collected two types of material;

sediment samples and organic fragments for TL

and AMS radiocarbon dating. For Ton Ngoon

segment, we collected seven sediment samples

and four organic fragments (as shown in the

sampling location in Fig. 2). In the case of the

Ban Mai segment, the organic fragments for

radiocarbon dating were insufficient. We, therefore,

collected six well- preserved sediment samples

from both sides of the identified fault, for a TL

dating based approach (see the location of collected

samples in Fig. 3).

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J. App. Sci. Res., 5(2): 168-180, 2009

Fig. 2: Log stratigraphy of the west wall of Ton Ngoon trench showing characteristics of the sediment deposits,

faults, and the sampling points for dating analysis.

3.1 Ams-Radiocarbon Dating: For AMS-radiocarbon

dating, we stripped the outer parts and conducted a

routine cleaning method to prevent possible

contamination of the organic samples. The treated

samples were over 20 g in dry weight. The samples

were analyzed by the Accelerator Mass Spectrometry

Laboratory at the University of Arizona, USA, and the

results are summarized in Table 1.

3.2 Thermoluminescence-TL Dating: For the TL

dating approach, we adapted the quartz inclusion

technique for operating the sample treatment (Fig. 4).[15]

The bulk of the sediment sample was dried to evaluate

the water content, and then ~300 g by weight sieved

through 840 um mesh filters for annual dose

evaluation. Thereafter, a grain size fraction of 74 to

250 um was extracted by re-sieving from the remaining

sample portion, and then immersed in hydrochloric acid

(35 %) for at least 30 minutes to remove the carbonate

and organic matter, followed by 24 % hydrofluoric acid

to dissolve feldspar and to etch the alpha-affected

surface of quartz . Magnetic minerals were finally[11]

eliminated by an isodynamic magnetic separator. X-ray

Diffraction (XRD) analysis was used to detect feldspar;

and hydrofluoric acid etching was repeated until the no

feldspar peak was detected by XRD, which was taken

to represent the complete removal of feldspar

components.

Artificial irradiation of the section was performed

by a beta source. Ten hours of natural sunlight

bleaching is assumed to be complete bleaching to the

residual level . The regeneration technique is[12] [13]

suggested for evaluating the equivalent dose for

avoiding the saturation phenomena of TL signals that

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J. App. Sci. Res., 5(2): 168-180, 2009

Fig. 3: Log stratigraphy of the south wall of Ban Mai trench showing characteristics of the sediment deposits,

faults, fracture and the sampling points for dating analysis.

Table 1: AM S radiocarbon dating of organic fragments collected from the Ton Ngoon trench.

Sample No. Depth (cm) Fraction dated Weight (g) fraction of C-13 C-14(Year BP) C-14 calibrated

modern (Fm ) (Year BP)

C1 70 Organic residue 1.86 0.7999 ± 0.0035 -24.69 1,793 ± 35 1,758-1,828

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are frequently found when using the total bleachtechnique . TL-intensity measurements were[ 1 2 ]

determined using the high quality TL instrument atAkita University, Japan, and the results of equivalentdose portion are summarized in Table 2.

For the total dose rates (annual dose), theconcentrations of U, Th and K were analyzed from thegamma spectrometry at Akita University. Both beta andgamma dose rates were calculated using the formulaeof Bell . A small correction for water content has[14]

been applied to both the beta- and the gamma-measured dose rates according to Zimmerman . A[15]

beta attenuation factor of 0.84, caused by the grainsize, was considered appropriate to allow for the betadose attenuation within the selected size of quartzgrains , whilst the cosmic ray contribution was taken[11]

as 0.15 mGy/yr . The annual dose and TL dating[1 6]

results are summarized in Table 2.

3.3 Comparison of Tl Dating and Other ScientificDating: The slip rates and recurrence periods in faultzones can be determined if the deformed deposits arereliably dated . For the Ton Ngoon trench, AMS[17 ]

radiocarbon and TL ages can be used to cross checkeach other. However, for the Ban Mai trench there wasa lack of independent age control with only TL dataavailable for aging. Therefore, in order to test thereliability of the TL dating at the Ban Mai trench, wecompared the TL and AMS radiocarbon derived agesfrom the Ton Ngoon trench. These results, togetherwith other widely accepted aging methods fromselected samples of the same sedimentary layers fromvarious places in Thailand, were compared (Table 3).The added data consists of sediments which are relatedto the fault along the active Mae Chan Fault ,[5]

sediment samples from the Mae Hong Son landslide,subsidence-prone area , brick samples from the Thung [18]

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J. App. Sci. Res., 5(2): 168-180, 2009

Table 2: TL dating of sediment samples collected from the Ton Ngoon and Ban M ai trenches.

Sample No. U(ppm) Th(ppm) K(%) Water content Annual dose Equivalent dose TL ages

(%) (Gy/ka) (Gy) (Year BP)

Ton Ngoon segment

T1 1.59 6.37 1.25 10 3.27 4.90 1,500 ± 20

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T2 1.66 7.76 1.26 15 3.36 6.72 2,000 ± 110

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T3 1.84 6.88 1.30 15 3.38 11.83 3,500 ± 80

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T4 2.10 8.45 1.25 12 2.30 8.88 3,868 ± 649

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T5 2.27 8.41 1.34 15 2.29 9.52 4,158 ± 1,838

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T6 1.40 6.91 1.20 22 2.84 11.93 4,200 ± 160

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T7 1.78 7.67 1.40 19 3.41 20.80 6,100 ± 170

Ban M ai segment

M 1 2.60 11.92 1.42 14 4.75 18.05 3,800 ± 140

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M 2 2.38 12.93 1.46 14 4.83 22.70 4,700 ± 450

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M 3 2.65 13.47 1.60 10 4.52 28.47 6,300 ± 900

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M 4 2.71 13.29 1.28 10 5.14 40.09 7,800 ± 750

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M 5 2.64 16.55 1.25 13 5.46 218.40 40,000 ± 3,000

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M 6 2.51 15.94 1.50 10 5.67 102.06 18,000 ± 2500

Fig. 4: Simplified flow chart illustrating the laboratory analysis applied in TL dating.

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J. App. Sci. Res., 5(2): 168-180, 2009

Table 3: Geochronological comparison between TL dating and the other scientific dating in Thailand.

Location M aterial TL technique Age(yr) Error(yr) Ref. M aterial Other methods Age(yr) Error(yr) Ref.

[1] Se R 765 200 1 Ch P-AMS 515 - 1

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[1] Se R 985 150 1 Ch C-14 950 90 1

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[1] Se R 3,700 500 1 Ch P- AM S 3,745 - 18

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[2] Se R 242 - 1 Ch C-14 230 - 1

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[3] Sh TB 37,200 5,000 9 Bi AM S 43,480 50 38

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[3] Sh TB 38,400 6,000 9 Bi AM S 43,480 50 38

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[3] Sh TB 38,600 5,000 9 Bi AM S 43,480 50 38

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[4] Se R 9,980 120 18 Bo AM S 12,100 60 18

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[4] Ca R 13,422 - 18 Ch AM S 13,160 75 18

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[4] Se R 22,257 - 18 Sh AM S 22,150 60 18

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[5] Se R 2,980 180 18 Ch AM S 2,870 80 18

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[6] B TB 538 15 33 Ch C-14 455 85 37

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[6] St TB 519 17 33 Ch C-14 455 85 37

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[7] Te TB 745,000 55,000 35 Te Ar 770,000 20,000 34

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[8] Te TB 650,000 160,000 36 Te K 725,000 25,000 31

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[9] Se R 4,158 1,838 *** Se TB 5,011 1,403 ***

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[9] Se R 3,868 649 *** Se TB 3,690 1,011 ***

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[9] Se TB 3,690 1,011 *** Se R 3,500 180 5

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[9] Se R 3,868 649 *** Se R 3,500 180 5

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[9] Se R 4,158 1,838 *** Se R 4,200 160 5

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[9] Se TB 5,011 1,403 *** Se R 4,200 160 5

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[9] Se R 3,868 649 *** Ch AM S 4,030 39 5

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[9] Se TB 3,690 1,011 *** Ch AM S 4,030 39 5

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[9] Se R 6,100 170 5 Ch AM S 5,882 42 5

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[9] Se R 2,000 110 5 Ch AM S 1,940 35 5

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[9] Se R 1,500 20 5 Ch AM S 1,793 35 5

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[9] Se R 3,500 180 5 Ch AM S 4,030 39 5

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[10] B Ad 886 194 12 Ta Re 1,150 50 32

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[10] B R 1,080 263 12 Ta Re 1,150 50 32

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[10] B Ad 712 282 12 Ta Re 1,150 50 32

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[10] B R 818 85 12 Ta Re 1,150 50 32

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[10] B Ad 994 394 12 Ta Re 1,150 50 32

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[10] B R 1,142 119 12 Ta Re 1,150 50 32

Description of the abbreviation in table 3

Ref. = References;

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J. App. Sci. Res., 5(2): 168-180, 2009

*** = Result from Ton Ngoon segment

M aterial

Dating method

AM S = AM S radiocarbon dating K = K/Ar dating Re = Relative age dating

Tuk archaeological site, Phang Nga province , and12

samp les fro m the excava ted land fi l l a rea ,

Samutphrakarn province .[9]

The TL ages were plotted against the AMS

radiocarbon and other scientific ages from the nearest

stratigraphic level. If AMS and TL dates corresponded

exactly, they would fall on the black solid straight line

(Fig. 5). Indeed, there was found to be a very strong

almost exact linear correlation between TL and AMS

radiocarbon ages (Fig. 5c). In the case of the whole

comparison, the TL ages have been entered as obtained

and the corresponding AMS - TL age has been

interpolated. It therefore seems clear (Fig. 5) that the

TL-derived ages are congruent with the other aging

methodologies as a whole, whilst this strong agreement

between TL and other dating methods (R2 =0.92 -

0.99) implies that TL dating is a valid and powerful

dating method to age samples in both geological and

archaeological applications. Consequently, we suggest

that the TL dates evaluated from the Ban Mai trench

are also likely to be trustworthy and meaningful for the

evaluation of the earthquake parameters in the next

section.

3 .4 Earthquake H azard P ara meters from

Paleoseismological Investigation: For seismic hazard

analysis, the necessary and required information which

can be evaluated from paleosiesmic investigations

consists of the maximum earthquake magnitude ( max),m

frupture area (A ), slip rate (S), and recurrence interval

of the large earthquake (RI).

mm ax, To determine the possible we utilized the

relationship between moment magnitude (Mw) and fault

rupture length at surface (SRL) as per equation 1 .[19]

The SRL used for the Mw evaluation is taken from the

total length of Ton Ngoon and Ban Mai fault segments.

After that, we also evaluated the Af by using the

empirical relationship between the obtained Mw (from

equation 1) and Af 19 in equation 2.

Mw= 5.08+1.1 6log(SRL) (1)

fMw= 4.07+9.8log(A ) (2)

where Mw is the moment magnitude, SRL is the

surface rupture length of fault (km), and Af is the

rupture area of the fault (km ) 2

Based on satellite image interpretation, the

morphotectonic data show that Ton Ngoon and Ban

Mai segments have a SRL of nine and 29 km,

respectively. As a result, the Ton Ngoon segment can

generate an earthquake with a maximum Mw of around

6.2 and an estimated Af around 1.6 km .2

For the Ban Mai segment, we estimated the

maximum earthquake of up to Mw 6.8 with an Af of

around 1.9 km .2

The slip rate has been defined as the rate of slip

of a fault averaged over the time period 20 involving

earthquakes. The slip rate can be estimated using the

cumulative offset of dated deposits. Assuming both that

the slip rate of a fault is constant over the period of

observation and that there is no creep, then the slip

rate is a linear function of displacement.

In this study, the fault (F2 in Fig. 2) exposed in

the Ton Ngoon trench shows the obvious offset

sediment layer of about 30 cm. Based on our dating

information, the estimated time span of elastic rebound

for the single earthquake (F2 in Fig. 2) is about 2,600

years (ca. 3,500-1,800 years). Consequently, the rate of

fault movement at the Ton Ngoon trench is ~0.18

mm/yr. In the Ban Mai trench, the vertical offset of

sediment layers related with the fault is 15 cm and the

slip time is 2,500 years (ca. 6,300-3,800 year), yielding

a calculated slip rate of ~0.06 mm/yr.

We derived the large earthquake recurrence period

of the Ton Ngoon segment from evaluation of the

chronological dates of the two recent earthquake

events. Based on these earthquake events being 3,500

and 1,800 years ago for F1 and F2, respectively, the

recurrence interval of the Ton Ngoon segment can be

estimated around 1,700 years (ca. 3,500-1,800 years).

However, with only a single recent evident earthquake

in the Ban Mai trench, this is in-sufficient to estimate

the large earthquake recurrence interval.

4. Seismicity Investigation: Although earthquake

catalogues cover very short time periods compared to

paleoseismological data, the instrument recorded

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J. App. Sci. Res., 5(2): 168-180, 2009

Fig. 5: Calibration graph showing the relationship between TL ages (ka) and other scientific ages (ka) from Table

3.

earthquake records still provide indispensable data for

seismic hazard analysis. In this study, we evaluated

several earthquake parameters that are required for

seismic hazard analysis using earthquake records in the

catalogues. The evaluation of the seismicity (earthquake

catalogue) data was based on a modification to the

published method , as follows: [21]

- Firstly, we collected earthquake records from

various earthquake catalogues such as Incorporated

Research Institutions for Seismology (IRIS),

Harvard University, Centroid - Moment Tensor

Project (CMT), and the Thai Meteorological

Department (TMD). Thereafter, a composite

earthquake catalogue was constructed and

overlapping earthquake events were eliminated.

- Next, because the composite earthquake catalogue

contained various earthquake magnitude scales (i.e.

mb, Ms, ML and Mw) all of these magnitude

scales were converted to the moment magnitude

(Mw) which represents physical properties of the

earthquake source and avoids the "saturation

phenomenon" at large seismic moments , [22]

- For the seismic hazard analysis, independent

earthquake (main shock) events must be served .24

To satisfy this requirement, the obtained

earthquakes catalogue needed to be de-clustered by

removing foreshocks and aftershocks. De-clustering

the foreshock and aftershock sequences was

performed using the assumption of Gardner and

Knopoff .[25]

- Thereafter, the independent earthquakes located

within the Lampang and Phrae Basins were

identified to represent the earthquake activity in

the respective faults in the LTFZ, including the

PFZ (Fig. 1a).

4.1 Earthquake Hazard Parameters from Seismicity

Investigation: In this section, earthquake hazardparameters, including the earthquake activity (a and b

values) in the Gutenberg-Richter (G-R) relationship,and the minimum earthquake magnitude (Mmin), were

evaluated for the LTFZ including the PFZ using theseismicity data from the completeness earthquake

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J. App. Sci. Res., 5(2): 168-180, 2009

catalogue. The earthquake activity of the LTFZ andPFZ were quantified using the G-R relationship .[26,27]

(equation 3), which is a key element in estimating theprobability that an earthquake with magnitude M or

larger will occur within a specific time interval.

log( n(M)) = a - bM (3)

where n(M) is the annual frequency of earthquakeswith magnitude M and larger. The constant values a

and b represent the entire seismicity rate and seismicitypotential, respectively.

We estimated the optimal a and b values of theLTFZ and PFZ, those that yield the relationship

between the observed log(n(M)) and M, by using theZMAP software . As shown in Fig. 6a, the LTFZ[21]

indicated a and b values of 2.51 and 0.61, respectively,whereas the PFZ (Fig. 6b) revealed a and b values of

2.6 and 0.77, respectively. For the minimum magnitude (Mmin), we used the

cmagnitude of completeness (M ), evaluated from the G-R relationship, to represent the Mmin for seismic

hazard analysis. The term Mc is defined as themagnitude above which all earthquakes are considered

to be fully reported (Fig. 6). Mmin values weredetermined at Mw~0.3 and 1.2 for LTFZ and PFZ,

respectively.

4.2 Temporal B Value Variation: A decrease orincrease in the b value is interpreted as increased or

decreased stress, respectively, before an approachingearthquake event . As such, the inverse correlation[28,29]

between the amount of stress accumulated in thehypocentral area and the b value is obviously of

particular interest in the forecasting of earthquakes. For the seismicity investigation in this study, we

investigated the b value variation depending on time

(b(t)), A previous study using windows with 50 events

of earthquake, and corresponding increments of five

events, to investigate the b(t) association with the large

earthquakes (Mw 7-9) in the Andaman subduction

region, proposed that large earthquakes occur when b

decreases by more than 0.3-1.0, and suggested that the

variation of the b value can be used as a medium-term

(months- years) earthquake precursor . In this study,[30]

we adapted this method to medium to small sized[30]

earthquakes (Mw 3-4) for finding the relationship

between b value variation and earthquake occurrence.

After several preliminary trails, we decided to

investigate the temporal variations of b values, b(t),

using sliding time windows containing 30 events with

five event shifts at a time. The data are presented in

Fig. 7 which displays the calculated b values as a

function of time for the LTFZ, and reveals three

obvious b value drops (down to 1-1.2) when the Mw

3-4 earthquakes occurred. The first one is in January

1997, the second one during the later half of 1998-

1999 and another in August 1999. We cannot define

whether or not the Mw 4 earthquake that occurred in

May 2000 is caused by the last drop in the b value

because the numbers of earthquake records with time

are reaching to the end of the available earthquake

catalogue.

conclusions: We evaluated the earthquake hazard

parameters in the LTFZ for seismic hazard analysis.

Both paleoseismological and seismicity data were

investigated. We successfully applied multiple dating

methods including TL and AMS radiocarbon dating, for

determination of the geochronological information

associated with paleo-earthquake throughout the slip

rates and recurrence period of the two investigated fault

segments.

Two obvious earthquakes have been identified on

the Ton Ngoon fault segment from detailed trench

investigation which, by TL and AMS radiocarbon

dating, were likely to have occurred 3,500 and 1,800

years BP, with a recurrence interval for large

earthquakes of 1,700 year and an estimated rate of

fault slip of around 0.18 mm/yr for the more recent

paleo-earthquake.

In the Ban Mai trench, only one palaeo-

earthquake event was defined. The slip rate of this

fault is about 0.06 mm/yr based on age information

from TL dating alone. However, the age comparison of

TL with other dating methods reveals a good positive

correlation, with the linear regression of about 0.92-

0.99. This strongly suggests that the TL dating method

is consistent with that of other scientific dating

methods and is sufficient to constrain the chronological

data in both geological and archaeological applications.

We, therefore, summarized that the obtained TL dates

at the Ban Mai trench can provide a close age estimate

on palaeo-earthquake events.

Comparison of the paleoseismological data of the

LTFZ reported here, with that for the adjacent area, the

PFZ , reveals that the LTFZ has a higher potential3

seismic activity than the Phrae side in the sense of a

higher slip rate and a shorter recurrence period. In

addition, the lower a and b values in the LTFZ

compared to those for the PFZ (Fig. 6) implies that the

LTFZ has a higher potential (seismic hazard) to

generate an earthquake than the PFZ in the future.

Finally, by using the suitable sliding time windows

containing 30 events with five event shifts at a time,

the b(t) shows a prominent drop in the b value to 1-1.2

before each earthquake with a magnitude Mw 3-4

occurred. We conclude that, this standard condition is

successful for analyzing the b(t) in the LTFZ and can

be applied for forecasting the occurrence of small to

intermediate sized earthquakes in other specific areas.

177

J. App. Sci. Res., 5(2): 168-180, 2009

Fig. 6: Gutenberg-Richter relationships of earthquake events within a) the Lampang-Thoen and b) the Phrae fault

zone. Triangles indicate the number of earthquakes in each magnitude, whilst squares represent the

cumulative number of earthquakes equal to and larger than each magnitude. Solid lines are the best fit.

Mc is the magnitude of completeness.

Fig. 7 a): Temporal variation of b values. The graphs were calculated by means of sliding time windows

comprising 30 events moving five events at a time. The heavy line indicates mean b values whereas

the dashed lines show the standard deviation. b) Temporal variation of Mw earthquake. The grey lines

mark the relationship between the occurrence of earthquakes with a Mw of more than 4 and the

decreasing b values.

178

J. App. Sci. Res., 5(2): 168-180, 2009

ACKNOWLEDGEMENTS

Our sincere thanks go to Akita University for

support in the equipment section and providing

convenience and comfort during the work in Japan.

Field mapping and trenching of the Ton Ngoon trench

are partially supported by Thailand Research Fund

(TRF). We thank the Office of Atomic Energy for

Peace (OAEP), Thailand, for the section of artificial

irradiation.

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